K9AY:
> This is easily explained by uncertainty in choosing an equivalent
diameter "solid wire" for
the 25G tower (I used 5 inches).
That sounds too small. Here's the formula I used to model
my 160 vertical made of Rohn 45:
http://lists.contesting.com/archives//html/Antennaware/2002-05/msg00001.html
Equivalent Cylindrical Diameter = 2 * CUBEROOT [(D * F^2)/2]
where D = tube diameter and F = Face width. For Rohn 45, D = 1.25"
and F = 18", so cylindrical diameter equivalent = 11.7446"
This turned out to be very accurate in predicting both the tap point
and capacitance needed for my shunt fed tower (and later a 3-el
parasitic array using the tower as the driven element). I don't have
the tube diameter for Rohn 25 handy to actually go through the
formula but think the equivalent diameter might be closer to 8" (just
mentally scaling the result above).
73, Bill W4ZV
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